I stumbled upon a useful site earlier today that's worth sharing. Called SimilarWeb, this small Firefox (and soon Internet Explorer) add-on sits on the side of your browser and pulls up sites that are similar to the one you're currently on.
It works remarkably well--at least with major sites. For example, visiting YouTube brings up a long list of other video hosts. The same went for social news sites like Digg, Reddit, and Delicious. You can scroll through these and open them up in new tabs, or pick from one of the tags SimilarWeb believes to be related to that page. This will pull up an entirely new list of places it thinks you should visit.
What makes the service shine is that users can re-arrange the lists and submit new sites that are not yet in SimilarWeb's index. There are thumbs up and down buttons which can raise or lower a site's standing on the list. Down-voting any site will actually remove it from the list. As a result, if users continue to vote the list gets more accurate.
There is a notable downside to using this extension: your browsing performance will take a hit. For some reason it needs to load its own results before it loads the actual page, which in my case meant waiting an extra few seconds when visiting a new site. That might be a deal killer for some, although it can easily be avoided by learning the keyboard shortcut that dismisses it from running in the sidebar. You can also pull up the results from a drop down menu next to your browser's address bar.
SimilarWeb would make a good companion for the now toolbar-free StumbleUpon, which actually learns from your browsing habits to give you pages it thinks you'll like. Combined with this you'd get another avenue of exploration.
Here's an explanatory video from the site's about page. Worth noting is that you don't need to have your volume on.
SimilarWeb 3 from Similar on Vimeo.